Method of fabricating electrode catalyst layers with directionally oriented carbon support for proton exchange membrane fuel cell

ABSTRACT

A membrane electrode assembly (MEA) of the invention comprises an anode and a cathode and a proton conductive membrane therebetween, the anode and the cathode each comprising a patterned sheet of longitudinally aligned transition metal-containing carbon nanotubes, wherein the carbon nanotubes are in contact with and are aligned generally perpendicular to the membrane, wherein a catalytically active transition metal is incorporated throughout the nanotubes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/368,116, filed on Mar. 3, 2006, which claims the benefit of U.S.Provisional Application Ser. No. 60/684,864 filed May 26, 2005, each ofwhich is incorporated herein by reference in its entirety.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention pursuant toContract No. W-31-109-ENG-38 between the U.S. Department of Energy andThe University of Chicago and/or pursuant to Contract No.DE-AC02-06CH11357 between the United States Government, U.S. Departmentof Energy and UChicago Argonne, LLC representing Argonne NationalLaboratory.

FIELD OF THE INVENTION

This invention relates to the preparation of aligned carbon nanotubeswith transition metal catalyst sites longitudinally spaced therealongand includes subject matter related to U.S. Provisional Application Ser.No. 60/692,773, filed Jun. 21, 2005, and non-provisional ApplicationSer. No. 11/368,120, filed on Mar. 3, 2006, each of which isincorporated herein by reference in its entirety.

BACKGROUND INFORMATION

The proton exchange membrane fuel cell (PEMFC) continues to benefit fromintense development efforts for its potential application in automobilesand distributed power generation due to a number of inherent advantagesincluding high efficiency, low noise and chemical emissions, and lowoperating temperature. A PEMFC typically consists of a membraneelectrode assembly (MEA), gas diffusion electrode (GDE) layers andbipolar plates. The MEA consists of an anode, a cathode and a membraneelectrolyte and is the key element of the fuel cell. During PEMFCoperation, hydrogen is electro-oxidized at the anode. The proton thusproduced is transported through the electrolyte and is combined with anoxide ion formed through the reduction of oxygen at the cathode. Atpresent, the electrode catalyst materials used at the anode and thecathode are primarily platinum supported over amorphous carbon. Sinceplatinum is a precious metal with limited supply, reducing its usagewill result in significant reduction in PEMFC cost for thecommercialization. One of the contributing causes of high precious metalusage is inefficient utilization of the precious metal the presentelectrode catalyst preparation method. Generally, the MEA preparationsteps involve catalyst synthesis, ionomer/catalyst ink preparation andcasting catalyst ink onto the membrane electrolyte. The catalystsynthesis usually is accomplished through a wet chemical process inwhich the precious metal precursor is deposited over a high surface areacarbon followed by a chemical reduction. The electrode catalyst thusprepared has highly dispersed metal crystallites distributed throughoutthe surface of carbon black. The catalyst is subsequently mixed with apolymer solution, known as ionomer, to form the ink. The ink is thencast over each side of the polymer electrolyte through a hot-pressingmethod to form the MEA. An intrinsic limitation to this approach is thata significant amount of catalyst is embedded underneath of the polymermatrix during the hot-pressing, rendering it inaccessible to gas flow.Therefore, these catalyst can not participate in the electro-chemicalreaction are thus considered under utilized.

A GDE is another key component in a PEMFC. The GDE is typically made ofcarbon paper or cloth treated with a hydrophobic coating. A GDE ispacked at each side of the MEA between electrodes and the bipolar platesto improve the electric conductivity, humidity control as well asreactant gas distribution. A GDE adds additional manufacturing cost andcomplexity to a PEMFC fabrication. The bipolar plate in PEMFC is made ofcorrosion resistant, electric conducting materials such as graphite orsurface treated stainless steel. Complicated gas flow channels, known asthe flow field, are often required to be embossed on the bipolar platesurface to distribute the hydrogen or oxygen uniformly over each side ofthe MEA. The bipolar plate also electrically connects the adjacent fuelcell modules to form the PEMFC stack. Construction of a flow field on abipolar plate adds cost and complexity to the PEMFC fabrication process.

Wilson and Gottesfeld summarized the conventional method of preparingmembrane electrode assemblies for a PEM fuel cell as disclosed in Wilsonand Gottesfeld, Journal of Applied Electrochemistry 22, (1992) pp. 1-7,incorporated herein by reference, discloses the method of forming thinfilm catalyst layers for MEA by preparing ink containing amorphouscarbon supported precious metal, followed by applying the ink andhot-pressing. Grot and Banerjee U.S. Pat. No. 5,330,860 incorporatedherein by reference, further describes a method of preparingelectro-catalyst ink and frication of MEAs with such ink. Harada U.S.Pat. No. 5,399,184 incorporated herein by reference, discloses a methodof making MEAs and a fuel cell assembly with gas diffusion electrodes(GDE). Wilkinson et al. U.S. Pat. No. 5,521,018 and disclosed herein byreference, discloses a design of bipolar plate with embossed fluid flowfield that has functions of conducting electricity and directing thereactant gas flow.

The inventive method is different from these conventional approachesbecause: (a) there is no need to prepare a carbon based catalyst inkthrough mixing before transferring the films to MEAs. Aligned carbonnanotube layers are transferred to a membrane electrolyte with thenanotube orientation and pattern remaining intact: (b) the alignedcarbon nanotubes in the present invention have excellent electricconductivity and hydrophobicity, therefore rendering the application ofGDE unnecessary; (c) a gas flow field pattern can be optionally builtduring the preparation of the aligned carbon nanotube bundles accordingto the invention; therefore, there is no need for embossing the flowfield in bipolar plate thus minimizing the manufacturing cost.

A recent patent application by McElrath et al. U.S. Publication No.2004/0197638 A1, incorporated herein by reference, discloses a method ofpreparing a membrane electrode using carbon nanotube materials includingthe steps of suspending nanotubes in solution, filtering nanotubes toform thin mat or dried catalyst ink over a membrane electrolyte. Thisinvention is different in the following aspects: (a) carbon nanotubesare transferred directly to the membrane electrolyte without liquidsuspension or filtration; and (b) carbon nanotube bundles are aligned inthe same direction with optional 3-dimensional pattern whereas thecarbon nanotubes in the prior art can not be aligned due to limitationsin the method of preparation.

Another recent patent application by Toops, U.S. Publication No. US2004/0224217 A1, disclosed herein by reference, discloses a method ofpreparing aligned carbon nanotube for MEA fabrication by pyrolyzinghydrocarbons inside of porous channels of an anodized alumina template,followed by dissolving the alumina with acid. This invention is superiorin the following aspects: (a) the aligned carbon nanotubes in thisinvention are prepared through growth over a substrate plate throughchemical vapor deposition without the need of an alumina template toguide the vertical alignment, obviating the cost of an alumina templateand the acid removal process; and (b) this method can produce carbonnanotube electrodes with three dimensional patterns as the result of thepreparation of substrate. After transferring on to the membraneelectrolyte, a pattern can be formed in which certain nanotubes arelower in height than the neighboring ones. One of such patterns is thestraight channel, as is shown in FIG. 6. These channels serve as thedistribution conduits for the reactant gas flows uniformly throughoutthe electrodes, which have advantage of replacing flow field in bipolarplate in the conventional design. In addition, the electrode preparedwith the aligned carbon nanotubes according to the invention has highernanotube density than the prior art. Because the inventive method istemplate free, nanotubes can grow closely and in contact with eachother, as is shown in FIG. 4. The prior art requiring alumina templatesdoes not have this flexibility.

SUMMARY OF THE INVENTION

This invention relates to a new method of preparing a membrane electrodeassembly (MEA) for a PEMFC that reduces precious metal usage, eliminatesthe need for GDE and simplifies the design and fabrication of bipolarplates. The inventive method consists of multiple steps includinggrowing template-free aligned carbon nanotubes, transferringdirectionally aligned nanotubes onto the surface of a polymerelectrolyte, and fabricating a fuel cell with flow field-free bipolarplates. This new method also leads to a new PEM fuel cell design inwhich either or both anode and cathode are made of aligned carbonnanotubes with a portion of the nanotubes embedded in a protonconduction polymer and another portion exposed to reactant gas flowcontacting the bipolar plates directly. The aligned nanotubes can alsobe built optionally with a pre-designed flow field pattern. MEA designeliminates the need for GDE and the necessity of embossing flow fieldson the surface of bipolar plates.

An important object of the invention is to prepare a three-dimensional,aligned carbon nanotube with a pre-designed gas flow pattern, using asilicon or a silica transfer substrate with a 3D gas flow patternprepared through coating a layer of photo-sensitive polymer, followed byexposing ultraviolet light over the photo-mask, washing off theunpolymerized coating layer, acid etching of the substrate and removalof polymerized layer. Carbon nanotube bundles can be grown and form avertical 3D layer with the same pattern as that of the photomask. Bothpositive and negative photo-resist coatings, as is known in the art, canbe used for this purpose.

Another object of the invention is providing a metal substrate for thealigned carbon nanotube growth such as nickel and stainless steel, witha 3-dimensional pattern therein by machining such as milling, stamping,engraving as is known in the art.

Yet another object of the invention is to provide a three-dimensionalpattern ACNT having a pattern of a fluid flow field channel distributingreactant gas uniformly throughout the area defined by the nanotubebundles. Such flow field channels can have a wide variety of patterns,i.e. straight line, wavy line, serpentine line, and other shapes knownin the art.

Another object of the invention is to transfer precious metal basedelectrode catalyst material to the aligned carbon nanotubes through wetchemistry method or through vapor deposition method, as known in thefield of the art.

A further object of the invention is to incorporate highly dispersedcatalytically active transition metals into the aligned carbon nanotubeduring the chemical vapor deposition step with or without chemicallyassociated nitrogen.

A still further object of the invention is to transfer aligned carbonnanotubes from the substrate to a membrane electrode through a hot-pressmethod to produce a MEA with aligned carbon nanotube orientedperpendicular to the membrane surface.

Another object of the invention is to provide a MEA containing alignedcarbon nanotubes with 3-D pattern on one or both sides of theelectrolyte membrane and perpendicular thereto and may function as flowfields for reactant gases to improve the electric conductivity or thedistribution of gases. The aligned carbon nanotubes can be used as thesupport for electrode catalysts or have electrocatalytic function for anoxygen reduction reaction.

A final object of the invention is to provide PEM fuel cells wherein theinventive MEA remove the need of embossing a flow field on the bipolarplates. Furthermore, graphitic carbon nanotubes contact directly thebipolar plates with improved electric conductivity and remove the needof a gas distribution electrode (GDE). Such improvements simplify thePEM fuel cell manufacturing process and reduce the cost.

The invention consists of certain novel features and a combination ofparts hereinafter fully described, illustrated in the accompanyingdrawings, and particularly pointed out in the appended claims, it beingunderstood that various changes in the details may be made withoutdeparting from the spirit, or sacrificing any of the advantages of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of facilitating an understanding of the invention, thereis illustrated in the accompanying drawings a preferred embodimentthereof, from an inspection of which, when considered in connection withthe following description, the invention, its construction andoperation, and many of its advantages should be readily understood andappreciated.

FIG. 1 is a flow chart illustrating steps in the inventive process. FIG.2( a)-(j) are schematic representations of articles made during theprocess of producing a PEMFC.

FIG. 3( a)-(c) show SEM images taken after a nanotube bundle was removedfrom a substrate.

FIG. 4( a) and (b) show SEM image of the ends (a) of a nanotube bundleafter removal from a quartz substrate, and sides (b).

FIG. 5( a) and (b) are SEM images of a nanotube bundle removed from asubstrate (a) and transferred to an electrolyte membrane (b) showing thetransfer did not change the tube orientation.

FIG. 6( a) and (b) show three dimensional (3D) patterns formed bynanotube on a substrate (a) and transferred to a membrane (b) forming aflow pattern thereon.

DETAILED DESCRIPTION OF THE INVENTION

The method of preparing an electrode catalyst layer with a 3-dimensionalaligned carbon nanotube for a PEMFC can be described according to theprocess map shown in FIG. 1.

The first step involves the preparation of the surface of the substratefor nanotube growth. In the preferred embodiment of the invention, thesubstrate is made of silicon or silica (silicon dioxide) plates that aretolerant to temperature up to 1000° C. Specifically, the substrate plateis made by quartz. The preparation includes immersing the quartzsubstrate plate into a hot liquid bath containing a mixture ofconcentrated sulfuric acid (98%) and hydrogen peroxide solution (30%) atthe temperature below 100° C. After rinsing with deionized water, thesubstrate plate is further cleaned in an ultrasonic bath containingacetone for 30 to 60 minutes, before being dried at room temperature.After drying, the substrate is coated with a layer of photo-resistpolymer through a spin-on coating process. The photo-resist polymersused for this purpose can be either positive or negative, or any othertypes known in the art. An example of the negative photo-resist polymersolution for the coating process contains N-(hydroxymethyl)acrylamide,triphenylsulfonium triflate and polyvinyl alcohol (PVA) mixed in adeionized water and acetonitrile solution. After drying and curing in avacuum oven at mild temperatures around 80° C., a thin layer ofphoto-resist layer was formed on the surface of the glass substrate, asis shown in FIG. 2 a. In the preferred embodiment of invention, thecoating layer thickness ranges from about 0.5 to about 3 microns (μm).An example of the positive photo-resist polymer solution isdiazonapthoquinone and Novalac resin dissolved in propylene glycolmethyl ether acetate, such as that marketed under brand name ShipleyS1818. A similar drying and curing process is also needed for positivephoto-resist coating. Other substrate materials such as nickel,stainless steel, and other metals can be used as substrates. In the caseof metal substrate, a 3-dimensional pattern can be applied to thesurface of the substrate through method of machining such as milling,stamping, engraving as is known to those of ordinary skill in the art.

The second step involved forming a pattern polymer layer over thesubstrate surface through a photolithographic method, with a methodknown in the art, as described in L. F. Thompson et al., Introduction toMicro Lithography, 2d Ed., American Chemical Society, Washington, D.C.(1994), and incorporated herein by reference. A photo-mask with apreselected pattern, such as that of a gas flow field channel pattern,was placed over the substrate coated with a photo-resist layer, followedby the exposure of a light source, such as an ultraviolet lamp. Theoptical opaque region of the photo-mask blocked the illumination fromreaching the coated layer underneath while the unblocked region wasfully exposed, as is shown in FIG. 2 b. For a negative photo-resist, thepolymer coating in the exposed region undergoes a cross-linking processand become insoluble whereas the coating in the unexposed region iswashed away by the deionized water. In the case of positivephoto-resist, the coated layer exposed under the radiation becomessoluble in the developing solution and subsequently is removed. Theunexposed coating remains intact after the development. The gas flowfield channel can have a variety of patterns such as straight line, wavyline, serpentine line, and any other shapes that are desired.

The third step of the invention involves forming a three-dimensionalpattern on the substrate over the photo lithographed coating from step 2using an acid etching method. An acid solution, generally containing themixture of hydrofluoric acid and ammonia fluoride, is used to dissolveuncovered glass surfaces at ambient or slightly above ambienttemperature. The acid etching process did not stop until a predetermineddepth was reached, as that is shown in FIG. 2 c. In the preferredembodiment of invention, the depth ranged from about 1 micron to about50 microns.

An alternative embodiment of the invention in step 3 is to remove thepatterned polymer layer from 2^(nd) step through a carbonization stepwithout undergoing the acid etching process. The removal can beaccomplished by carbonizing the coated layer in dry air at an elevatedtemperature in the range of from about 400° C. to about 900° C. Theglass substrate thus prepared can also be used for growing alignedcarbon nanotube in step 5, as will be discussed later.

The fourth step of the invention involves removing the coated layer andre-cleaning the surface after acid etching process. The removal can beaccomplished by calcining the patterned substrate in dry air at thetemperature ranging from about 400° C. to about 900° C., followed by acleaning procedure similar to that discussed in the step one. A 3-Dsurface on the substrate is thus formed ready for aligned carbonnanotube growth, as is shown in FIG. 2 d.

The fifth step of the invention involves forming the aligned carbonnanotube over the prepared substrates from the above steps using achemical vapor deposition (CVD) method. The CVD process is carried outinside of tubular quartz reactor in a two-zone furnace in which thetemperature of each zone can be individually controlled. In oneembodiment of the invention, phthalocyanine containing transitionalmetals such as iron phthalocyanine (FePc) or cobalt phthalocyanine(CoPc) is placed in the first heating zone of the reactor which istypically operated from about 300° C. to 800° C. In the preferredembodiment, FePc is placed in 1^(st) heating zone operated from about500° C. to about 700° C. The iron phthalocyanine is vaporized at thesetemperatures and the vapor is carried by a flowing gas mixture of argonand hydrogen into the second heating zone of the reactor where thepatterned substrates are placed. The ratio of Ar/H₂ ranges from about10:1 to about 5:5 and the temperature of the 2^(nd) heating zone rangesfrom about 700° C. to about 1000° C. The phthalocyanine vapor pyrolyzesat these temperatures on the surface of the substrates and formsthree-dimensional aligned carbon nanotube bundles, as is shown in FIG. 2e. In addition to Fe and Co phthalocyanines, other transition metalcompounds such as nickel phthalocyanine (NiPc) can also be used.Furthermore, the organometallic compounds with the structure of atransition metal coordinated by four nitrogen from the surroundingpyrrolic nuclei can also used for this purpose. Such compounds includetransition metal porphyrins and their derivatives. Examples include ironporphyrin, cobalt porphyrio. Moreover, various mixtures of any of theabove are useful.

Another embodiment of the invention involves preparing aligned carbonnanotube using an organometallic compound and organic aromatics mixture.In the preferred embodiment, the organometallic compounds are transitionmetal phthalocyanines and metallocenes such as iron phthalocyanine andferrocene. The organic aromatics are xylene, toluene, benzene, pyridine,as well as others. The organometallic is generally fully dissolved inthe aromatics to form a liquid mixture. The liquid was injected into the1 ^(st) heating zone of the reactor with a temperature ranging fromabout 150° C. to 500° C. The injected mixture vaporized and mixedinstantaneously with an Ar—H₂ carrying gas and was carried into the2^(nd) heating zone of the reactor where the temperature was controlledat from about 600° C. to about 900° C. The mixture decomposed on thesurface of the patterned substrate and aligned nanotube bundles wereformed through pyrolysis. The aligned carbon nanotubes produced throughthe above described CVD processes have, in general, a multi-wallconfiguration which refers to a tube composed of multiple layers ofcylindrical graphic sheets. The graphitic carbon nanotubes thus preparedhad average diameters ranging from about 5 to about 100 nanometers andlengths ranging from about 3 to about 80 microns. The aligned carbonnanotubes on the surface of the substrate generally had similar lengths,as are shown in FIG. 2 e when the reaction temperature and gas mixturewere identical and when the substrate was treated according to step 4.

Yet another embodiment of the invention is to grow the patterned alignedcarbon nanotube following the carbonization process as an alternativestep to bypass step 3 and 4. In this case, a very thin layer ofamorphous carbon is formed after calcination of the coated layer. Suchamorphous carbon layer prohibits the growth of aligned nanotube whereasthe aligned nanotube will grow over the uncoated portion. Therefore, apattern nanotube layer can still be formed on the cleaned substratesurface whereas the amorphous carbon layer is free of nanotubes.

Yet another embodiment of the invention involves using a gas mixturecontaining ammonia instead of argon-hydrogen only. More specifically,the gas mixture consisted of NH₃, H₂ and Ar with the relative ratiosranging from about 1:4:10 to about 3:6:6 used as carrying gas for theapplication in both embodiments mentioned above, especially when FePcand ferrocene were selected for the nanotube growth. N—Fe—N bond mayhave been formed during the pyrolysis processes in the 2^(nd) reactorzone when NH₃ is present in the carrier gas. The functioned group wasembedded in the graphitic structure on the surface of carbon nanotubeslongitudinally spaced and had electro-catalytic activity for reducingoxygen to oxide ion in an oxygen reduction reaction (ORR) occurring at acathode during

PEM fuel cell operation as disclosed in M. Lefevre et al.,Electrochimica Acta 48 (2003), pp. 2749-2760, and incorporated herein byreference. The advantage of such approach is to reduce or to eliminatethe usage of platinum at the cathode with aligned carbon nanotubes withembedded N—Fe—N groups to significantly decrease the cost of a PEM fuelcell.

The sixth step of the invention involves catalyzing the aligned carbonnanotubes to functionalize them with electro-catalytic activity foreither anode or cathode application. Highly dispersed transitional metalor precious metal crystallites can be formed on the nanotube surfacethrough various wet chemical methods with the catalytic precursorsolution followed by the heat treatment, as known in the art. Thecatalyst precursors include the solutions of transitional metal orprecious metal salts, such as nitrate, sulfate, sulfite, chloride, etc.The methods of depositing metal solution over aligned carbon nanotubeinclude impregnation, wet-incipient, redox precipitation, etc. After thewet chemical treatment, the aligned carbon nanotubes were dried in dryair, followed by the reduction in flowing hydrogen at elevatedtemperature. The reduction temperature typically ranges from about 150to about 550° C. In a preferred approach, the reduction is carried inflowing hydrogen in the temperature range of from about 150 to about450° C. The metal ions were reduced to their zero valence state afterthe reduction and the catalyzed carbon nanotube can now be used aselectrode catalyst. In an alternative embodiment of the invention, theelectrode catalyst can also be coated over aligned carbon nanotubesthrough vapor deposition of a precious metal precursor in vacuum atelevated temperature, as is known in the art. Yet another alternativeembodiment to the current invention is to postpone the catalyzing stepafter transferring the aligned carbon nanotube to the polymerelectrolyte, as is shown in the process map in FIG. 1.

The seventh step of the invention involves transferring and connectingthe aligned carbon nanotubes to the surface of the membrane electrolyte.This is actually a two-stage process. The first stage involves applyinga solution of polymer that is compatible to the electrolyte material tothe top of the aligned carbon nanotube layer over the substrate. Forexample, if NAFION® ionomer is used as the membrane electrolyte, NAFION®ionomer solution is used. For the preferred embodiment, the NAFION®ionomer solution contains 5% polymer solid content. The applicationmethods can be spraying, painting, liquid dropping, or any other artrecognized method. After coating by polymer solution, the supportedcarbon nanotube tube is dried in air or in vacuum at the roomtemperature or up to about 150° C. A decal with a polymer layer coatedon the top of aligned carbon nanotube is formed at the end of the stage,as is shown in FIG. 2 f. The second stage of the process involvestransferring the decal to the polymer electrolyte using a hot-pressmethod. The hot-press is accomplished by applying pressure uniformlyover the substrate surface while maintaining the platen at elevatedtemperature. The pressure of the press ranges from about 1.5×10⁵ N/m² toabout 7.5×10⁶ N/m² and the temperature of the platen ranges from about180° C. to about 230° C. The pressing time ranges from about 3 to about8 minutes. In the preferred embodiment, the pressure of the press rangesfrom 3.5×10⁵ N/m² to 7.5×10⁵ N/m² and the temperature of the platenranges from about 200 to about 220° C. The hot-pressing can be carriedfrom both side of the membrane simultaneously, as shown in FIG. 2 g, orto be performed on individual side separately. The polymer coating fromthe first stage will be fused into the membrane during the hot-pressprocess. After the completion of hot-press, the decal/membrane assemblyis removed and cooled to room temperature. The MEA can subsequently bepeeled off from the substrate with the aligned carbon nanotube layerattached, as is shown in FIG. 2 h. In another embodiment, the peelingoff can be assisted after briefly immersing the substrate with nanotubecoated MEA in a fluoric acid solution. FIG. 2 i shows the top view ofthe MEA with channel aligned nanotube bundle patterns originated fromstep 2 and step 3. The channels pattern can be straight (as is shown inFIG. 2 i), wavy, serpentine, or any other desired shape that can begenerated through any applicable photolithographic method. The channelsserve as flow fields to guide the reactant gas at both the anode and thecathode; therefore the shape is determined by the required gasdistribution in a specific PEM fuel cell.

The aligned carbon nanotubes can be catalyzed after the MEA fabricationif they have not been previously catalyzed. The catalyzing method issimilar to that outlined in step 6 except that the hydrogen reductiontemperature is limited to less than about 200° C. to minimize thepotential damage to the polymer membrane.

The eighth step of the invention involves assembling the individualPEMFC by packaging the aligned carbon nanotube MEAs with the bipolarplates, as is shown in FIG. 2 j. In the current embodiment of theinvention, the bipolar plate contains a gas inlet and a gas outlet. Thebipolar plate does not need to have flow field patterns embossed on itscontact surface with the MEA, as the pre-designed aligned carbonnanotube pattern serves as the conduit for gas distribution. A portionof the aligned nanotubes protruding outward against the surface of thebipolar plate generates the electric contact between the MEA and thebipolar plate. The bipolar plate can be fabricated from a variety ofelectric conducting materials such as graphite, stainless steel andother metals with corrosion prevention treatment, if required.

Example 1

A thin quartz plate with dimension of ⅝ inch×1.5 inch×0.1 inch wascleaned in a solution containing 70 vol. % concentrated H₂SO₄ and 30vol. % H₂O₂. The solution temperature was maintained between 70° C. to100° C. After cleaning and rising with deionized water, it wasultrasonically cleaned in acetone for 30 min. The dried quartz plate wasready for the use as the substrate.

Example 2

A negative photo-resist solution was prepared by mixing the solution Aand B where A is made by dissolving 184 mg of triphenylsulfoniumtriflate in 12 ml water/acetonitrile solution followed by mixing with725 mg of N-(hydroxymethyl)acrylamide solution, and B was made bydissolving 2.029 g PVA in 20 ml water. The photo-resist solution wassubsequently filtered through a 0.1 μm filtration paper and used forspin-coating on the treated quartz substrate according to Example 1 witha high speed spin-coater (Model 1-EC101DT-R485, Headway Research, Inc.).A pipette was filled with the photo-resist solution. The solution wasdispensed over the quartz substrate in a droplet fashion while thecoater spun at 2500 rpm for 30 seconds. The coating was baked at 90° C.for 30 minutes inside of a vacuum oven. A photo-mask with linear strippattern was placed over the top of the coated quartz plate. Anultraviolet radiation with wavelength of 254 nm generated from an UVlamp (Mineralight, UVG-11) was applied to the coated plate through thephoto-mask for 5 minutes. The exposed plate was subsequently baked in avacuum oven at 130° C. for 8 min to complete the polymerization processfor the irradiated region. The plate was then immersed in deionizedwater after being cooled to ambient temperature and was rinsed for 5min. The unexposed portion of the coating was thus washed away. Thequartz plate with a coated polymer pattern was then dipped into ahydrofluoric acid (HF) solution made with 9 parts of water and one partof HF (49%) for 100 minutes. After rinsing and drying, the plate wascalcined in flow air at 500° C. for one hour before cleaned againaccording to the method described in Example 1.

Example 3

A positive photo-resist solution was obtained under the brand nameShipley S1818, that contains diazonapthoquinone and Novalac resindissolved in propylene glycol methyl ether acetate. The substrateprepared according to the Example 1 was first coated by a thin layer ofhexamethyldisilazane (HMDS) via a spin coater before being coated withthe photo-resist solution with the dry thickness about 2 microns. Afterbaking in vacuum oven at 90° C. for 30 minutes, a photo-mask with linearstrip pattern was placed over the top of coated substrate plate. Anultraviolet radiation with wavelength of 365 nm generated from an UVlamp (Ultra Violet Products, Model UVL-23RW) was applied to the coatedplate through the photo-mask for 15 seconds. The exposed substrate wassubsequently developed in an aqueous solution containing 5%tetramethylammonium hydroxide for 1 minute followed by rinsing indeionized water to wash away the exposed portion of the coating. Afterbaking at 130° C. for 30 minutes, the patterned quartz substrate wasetched in an acidic solution containing NH₄F and HF for about 1 hour.The unprotected portion of the quartz was etched to form a 3 dimensionalpattern. The substrate was then heated in air to 500° C. to burn off theremaining coating and cleaned again according to the steps described inExample 1. The substrate is now ready for growth of carbon nanotube.

Example 4

A quartz substrate prepared according to the Example 2 was placed insideof a tubular two-zone reactor where the temperature of each zone wascontrolled independently. 0.1 gram of iron phthalocyanine (FePc, 97%Aldrich) was placed in the first zone whereas the quartz plate is placedin the second zone. An argon-hydrogen (55%-45%) gas mixture entered thefirst zone with a total flow rate of 330 ml/min and carried thevaporized FePc into the second zone while the temperatures for the firstand second zone were controlled at 550° C. and 850° C., respectively.After 30 minutes reaction time, a layer of aligned carbon nanotube wasformed on the surface of the quartz substrate. Shown in FIG. 3 (a, b,and c) are SEM images taken after the a nanotube bundle was removed fromthe substrate.

Example 5

A quartz substrate prepared according to the Example 2 was placed insideof a tubular two-zone reactor as described in Example 4. One gram offerrocene was dissolved in 10 ml xylene and injected continuously intothe first zone of the reactor using a syringe pump. The liquid feed waspassed through a capillary tube and preheated to 180° C. to 225° C.prior to its entry into the furnace. At this temperature, the liquidexiting the capillary tube was immediately volatilized and swept intothe second zone, which was kept at 725° C., by a gas mixture ofargon-hydrogen-ammonia mixture (8:5:2) at the flow rate of 300 ml/min.After 30 minutes, aligned carbon nanotubes were formed on the surface ofthe quartz substrate. Shown in FIG. 4 is the top view image (a) of ananotube bundle after it was removed from the quartz plate, and sideimage (b).

Example 6

A layer of aligned carbon nanotube was prepared on a cleaned quartzsubstrate according to the procedure described in Example 4. The quartzplate with aligned nanotube was subsequently loaded on the spin coaterand layer of NAFION® ionomer solution (5 wt. % solid, Aldrich) wasspread over the nanotube layer by pipetting the solution while thecoater rotated at 1000 rpm. The process lasted about 30 seconds. Theprocess was repeated to apply a NAFION® ionomer solution on the secondaligned nanotube substrate. After the solution was dried completely, thetwo substrates were placed at top and bottom of a NAFION® membrane(NAFION® 115, Aldrich) with the aligned nanotube side facing the film.The assembly was then placed on the platen of a commercial heatedhydraulic press (Carver Laboratory) where the platen temperature wasmaintained at 200° C. A pressure was subsequently applied to theassembly at 7.5×10⁵ N/m² for 5 minutes before it was removed. After theassembly was cooled to ambient temperature, the quartz plates wereslowly peeled from the film. The aligned nanotubes were transferred tothe surface of the membrane electrolyte without changing the tubeorientation, as is shown in FIG. 5 (a and b).

Example 7

A quartz substrate prepared according to the Example 3 was placed insideof a tubular two-zone reactor using the same chemical vapor depositionmixture as described in Example 4. A uniform, three dimensional alignedcarbon nanotube layer was grown on both low and high areas of thesubstrate, as is shown by the ridges and planes in FIG. 6 a. Followingthe nanotube growth, NAFION® ionomer solution was applied to the top ofthe nanotube layer which was subsequently transferred to the NAFION®membrane through the hot press method described in Example 6. After thetransfer, the side of the nanotube layer previously attached to thesubstrate was now exposed to the surface. The high and low areas of thenanotube layer were now reversed, as is shown in FIG. 6 b.

While there has been disclosed what is considered to be the preferredembodiments of the present invention, it is understood that variouschanges in the details may be made without departing from the spirit, orsacrificing any of the advantages of the present invention.

1. A membrane electrode assembly (MEA) comprising an anode and a cathodeand a proton conductive membrane therebetween, the anode and the cathodeeach comprising a patterned sheet of longitudinally aligned transitionmetal-containing carbon nanotubes, wherein the carbon nanotubes are incontact with and are aligned generally perpendicular to the membrane,wherein a catalytically active transition metal is incorporatedthroughout the nanotubes.
 2. The MEA of claim 1 wherein the activetransition metal comprises Fe, Ni, Co, Cr, Mn, or a combination thereof3. The MEA of claim 1 wherein the active transition metal comprises Fe,Ni, Co, or a combination thereof.
 4. The MEA of claim 1 wherein thenanotubes form a three dimensional pattern of gas-flow channels on themembrane.
 5. The MEA of claim 1 produced by: (a) introducing acombination of a transition metal-containing carbon nanotube precursorand a gaseous mixture comprising an inert gas and a reducing gas into afirst reactor zone maintained at a first temperature and for a period oftime sufficient to vaporize the transition metal-containing carbonnanotube precursor, introducing the vaporized material to a secondreactor zone maintained at a second temperature that is higher than thefirst temperature and for a period of time sufficient to pyrolyze thetransition metal-containing carbon nanotube precursor, and growlongitudinally aligned carbon nanotubes with a catalytically activetransition metal incorporated throughout the nanotubes onto a substratepresent in the second reactor zone, wherein the substrate is patternedwith a template for forming longitudinally-aligned carbon nanotubesperpendicular to the substrate surface, and the carbon nanotubeprecursor is selected from the group consisting of a transition metalphthalocyanine, a transition metal porphyrin compound, a transitionmetal organometallic compound, or a combination thereof, optionally as asolution in an aromatic hydrocarbon solvent; (b) introducing transitionmetal or precious metal crystallites onto the surface of the nanotubes;(c) depositing a layer of an ionomeric polymer over the carbon nanotubesand drying the polymer to form a nanotube-containing polymeric decalhaving an outer polymeric side and an inner nanotube side in contactwith the substrate; (d) fusing the polymeric side of two such decals toboth sides of a proton-conductive membrane at an elevated temperatureand pressure; and (e) removing the substrates to thereby form the MEA.6. The MEA of claim 5 wherein the gaseous mixture comprises argon andhydrogen and optionally includes ammonia, and the transitionmetal-containing carbon nanotube precursor comprises Fe, Ni, Co, Cr, Mn,or mixtures thereof.
 7. The MEA of claim 5 wherein the temperature inthe first reactor zone is maintained in the range of from about 150° C.to about 700° C. and the temperature in the second reactor zone ismaintained in the range of from about 700° C. to about 1000° C.
 8. TheMEA of claim 5 wherein the material is maintained in the reactor zonesfor a period of time in the range of from about 5 to about 45 minutes.9. The MEA of claim 5 wherein the nanotubes form a three dimensional(3D) pattern of gas-flow channels on the membrane.
 10. The MEA of claim5 wherein the carbon nanotube precursor contains a transition metalselected from the group consisting of Fe, Ni, Co, and a mixture thereof.11. The MEA of claim 5 wherein the nanotubes are generally straight orinclude spiral or bamboo shaped or bellows shaped nanotubes.
 12. The MEAof claim 5 wherein one transition metal-containing nanotube decal of theMEA includes an oxidation catalyst and the other decal includes areduction catalyst.
 13. The MEA of claim 5 wherein a plurality of theMEAs are assembled into a proton exchange membrane fuel cell (PEMFC) byalternately stacking bipolar plates between the MEAs.
 14. A membraneelectrode assembly (MEA) comprising an anode and a cathode and a protonconductive membrane therebetween, one or more of the anode and thecathode comprising a patterned sheet of longitudinally alignedtransition metal-containing carbon nanotubes, wherein the carbonnanotubes are in contact with and are aligned generally perpendicular tothe membrane, wherein a catalytically active transition metal isincorporated throughout the nanotubes, and wherein the MEA is producedby: forming a sheet of longitudinally aligned graphitic nanotubes on asubstrate including a three dimensional pattern for aligning thenanotubes in a gas-flow pattern by chemical vapor deposition (CVD), andtransferring the so-formed sheet of nanotubes to a surface of a protonconductive membrane, wherein the chemical vapor deposition comprisespyrolysis of a transition metal-containing organometallic material toform nanotubes that include the transition metal within the nanotubestructure.
 15. The MEA of claim 14 wherein the active transition metalcomprises Fe, Ni, Co, Cr, Mn, or a combination thereof.
 16. A membranefor a proton exchange membrane fuel cell (PEMFC) comprising a protonconductive membrane that includes longitudinally aligned graphiticnanotubes generally perpendicular to the proton exchange membrane and incontact with a surface thereof, a catalytically active transition metalbeing incorporated within the nanotubes.
 17. The membrane of claim 16produced by forming longitudinally aligned graphitic nanotubes with acatalytically active transition metal in the nanotubes on a substrate bychemical vapor deposition (CVD), transferring the longitudinally alignedgraphitic nanotubes from or with the substrate to the proton conductivemembrane with the longitudinally aligned graphitic nanotubes beinggenerally perpendicular to the proton exchange membrane and in contacttherewith.
 18. The membrane of claim 16 wherein the active transitionmetal comprises Fe, Ni, Co, Cr, Mn, or a combination thereof
 19. Themembrane of claim 16 wherein the nanotubes form a three dimensionalpattern of gas channels on the surface of the membrane.